In this work we present the first spectroscopic results obtained with
the Southern African Large Telescope (SALT) during its
performance-verification phase. We find that the Sagittarius dwarf
spheroidal galaxy (Sgr) contains a youngest stellar population with
[O/H] ~ -0.2 and age t > 1Gyr, and an oldest population with [O/H]=
-2.0. The values are based on spectra of two planetary nebulae (PNe),
using empirical abundance determinations. We calculated abundances for
O, N, Ne, Ar, S, Cl, Fe, C and He. We confirm the high abundances of PN
StWr2-21 with 12 + log(O/H) = 8.57 +/- 0.02 dex. The other PN studied,
BoBn1, is an extraordinary object in that the neon abundance exceeds
that of oxygen. The abundances of S, Ar and Cl in BoBn1 yield the
original stellar metallicity, corresponding to 12 + log(O/H) = 6.72 +/-
0.16 dex which is 1/110 of the solar value. The actual [O/H] is much
higher: third dredge-up enriched the material by a factor of ~12 in
oxygen, ~240 in nitrogen and ~70 in neon. Neon as well as nitrogen and
oxygen content may have been produced in the intershell of low-mass
asymptotic giant branch (AGB) stars. Well defined broad WR lines are
present in the spectrum of StWr2-21 and absent in the spectrum of BoBn1.
This puts the fraction of [WR]-type central PNe stars to 67 per cent for
dSph galaxies.
Based on observations obtained with Southern African Large Telescope
(SALT).
E-mail: akniazev@saao.ac.za (AYK); a.zijlstra@manchester.ac.uk (AAZ)